By TOM PAULSON, P-I REPORTER

Published 10:00 pm, Tuesday, August 19, 2008

A 50-year-old King County woman appears to be the first known human to be infected by the West Nile virus in Washington this year, even though she was not made seriously ill.

The woman, according to state public health officials, was told of the viral infection after she donated blood. The blood center identified the virus during routine testing used to screen out donations that may be contaminated or carrying infectious agents before they get sent to the community blood supply.

"Most people who become infected with West Nile virus don't get ill," said Matias Valenzuela, spokesman for Public Health -- Seattle & King County. Valenzuela said it wasn't clear how, or where, the woman might have become infected and that state health officials were in charge of investigating the case.

"She had some minor symptoms but didn't go to the doctor," said Donn Moyer, spokesman for the state Department of Health. Because this case was identified through a blood donation rather than following the onset of illness, Moyer said it's even more difficult for experts to identify the actual source of the infection.

West Nile virus, named for a region in Uganda where it was first identified, has been spreading across the United States since it was first discovered on the eastern seaboard in 1999. Carried by mosquitoes, the virus infects birds and horses in addition to humans. In a minority of cases, it can cause severe illness such as brain swelling, seizures and death.

Washington has had an exceptionally low rate of human cases of West Nile infection, Moyer said. The first confirmed human case was in a Pierce County man in 2006. Two other people fell ill that year, he said, and there were no human cases last year. Experts estimate that less than one in 100 people who get infected will suffer any illness.

Six horses have been infected this year, and three of them either died or had to be killed, according to animal-disease experts at Washington State University. Public health officials track the spread of the virus in animals, which includes testing mosquitoes and dead birds.

"We've had more mosquito positive tests than ever this year but, for some reason, no positives in dead birds," Moyer said. It's not clear why, he said, and why Washington has had so many fewer human cases than most other states.

"We don't really have a solid explanation for that," Moyer said.

There's no question the virus is out there, he said, and little question it still poses a potential threat to both humans and many animals. The question, Moyer said, is why it has so far remained relatively quiescent in Washington as compared with nearly all other states.